1. Field of the Invention
The invention relates generally to electrophysiological (xe2x80x9cEPxe2x80x9d) catheters for ablating tissue, and more particularly to an improved tip electrode for an ablation catheter having multiple thermal sensors for improved measurement of electrode/tissue interface temperature.
2. Description of the Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities and restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the aberrant conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, two or more electrodes are introduced into the heart. The electrodes are oppositely charged and thus complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to increase the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. For therapeutic effectiveness, the ablation volume must extend a few millimeters into the endocardium and must have a surface cross-section of at least a few millimeters square. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C.
During ablation, portions of the electrodes are typically in contact with the blood, so that it is possible for clotting and boiling of blood to occur if those electrodes reach an excessive temperature. Both of these conditions are undesirable. Clotting is particularly troublesome at the surface of the catheter electrode because the impedance at the electrode rises to a level where the power delivery is insufficient to effect ablation. Additionally, too great a rise in impedance can result in tissue dessication and/or tissue explosion and thrombus formation within the heart, both of which are also undesirable. When any of these conditions arise, the ablation procedure must be stopped and the catheter removed and cleaned or replaced before the procedure can continue. Such delay in an ablation procedure is undesirable in that it may prove critical to the patient""s heath or survival.
Even though no significant amount of heat is generated in the electrodes themselves, adjacent heated endocardial tissue heats the electrodes via heat conduction through the tissue. As mentioned above, part of the active electrode will be in contact with the blood in the heart and if the electrode temperature exceeds 90-100xc2x0 C., it can result in blood clotting on the electrode. The application of RF energy must then be stopped. However, shutting the RF generator off due to the temperature rise may not allow sufficient time to complete the entire ablation procedure. Providing an ablation electrode capable of applying higher amounts of power for a longer period of time to ablate the damaged tissue to an acceptable depth is a goal of current ablation catheter electrode design. It has been found that higher power for longer time periods results in a higher probability of success of the ablation procedure.
To avoid clotting and blood boiling, RF ablation catheters for cardiac applications typically provide temperature feedback during ablation via a temperature sensor such as a thermocouple. In its simplest form, a thermocouple consists of two dissimilar metals joined together at one end called a xe2x80x9cbeadxe2x80x9d or junction, such as a conventional copper/constantan type xe2x80x9cTxe2x80x9d thermocouple. When the junction is heated a thermoelectric potential arises and can be measured across the unconnected ends. This is also known as the thermoelectric or Seebeck effect. This voltage is proportional to the temperature difference between the junction and the non-joined ends.
Many RF ablation catheters include a tip electrode for xe2x80x9cend-firexe2x80x9d ablation. The catheter is oriented such that the end of the tip electrode is in contact with the target tissue and RF energy is then applied. A tip electrode may contain a single end thermal sensor, typically located along the centerline of the tip, at or very near the apex of the tip electrode. The temperature sensor is thus in close proximity to the electrode/tissue interface when the tip electrode is oriented such that the apex of the electrode contacts the tissue during ablation, i.e. the xe2x80x9cend-firexe2x80x9d mode. If, however, the side of the tip contacts the tissue during ablation, i.e. the xe2x80x9cside-firexe2x80x9d mode, the radial distance from the end thermal sensor to the electrode/tissue interface is roughly equal to half the diameter of the tip electrode (e.g., approximately 1.167 mm for a 7 French diameter tip). There can therefore be a significant difference in the temperature measurements provided by the end thermal sensor depending on the orientation of the tip electrode.
During ablation, the temperature measured by a conventional ablation electrode positioned in the end-fire mode is closer to the actual tissue-interface temperature than the temperature measured when the electrode is positioned in the side-fire mode. The difference in measured temperature from actual tissue-interface temperature in the side-fire mode measurements is increased by high blood flow in the vicinity of the electrode. The high blood flow causes a steeper thermal gradient to arise within the tip electrode due to the increase in cooling of the electrode that the flow provides. This effect is commonly referred to as xe2x80x9cback-side cooling.xe2x80x9d
It is most advantageous for the thermal sensor to be located as close as possible to the electrode/tissue interface. However, in conventional catheters having a tip electrode containing only a single thermal sensor located at the end, a performance compromise between the side-fire and end-fire modes is commonly made in the design of the catheter. Additionally, tip electrodes provide other considerations in mounting temperature sensors. A tip electrode must be well anchored to the catheter shaft so that separation does not occur. Additionally, it must be thick enough to draw heat away from the tissue interface for cooling purposes yet not too thick so as to unduly increase the outside diameter of the catheter. Attaching a power lead to the tip electrode so that RF energy may be conducted by the electrode already adds one lead to the pair of leads connected to the sensor located at the end of the electrode.
Hence those skilled in the art have identified a need for improvement of overall temperature measurement in the tip electrode of an ablation catheter that can be used for both end-fire and side-fire ablation. Improved measurement capability can result in increased product efficacy, because the potential for a rise in electrical impedance, which typically prevents further delivery of RF energy, is reduced. The likelihood of thrombus formation is also reduced. It is also desirable to provide for an improved temperature feedback control system in an ablation energy delivery system configured as a closed loop system, with power being adjusted to maintain the temperature of the electrode/tissue interface below a threshold temperature. The present invention fulfills these needs and others.
Briefly, and in general terms, the present invention is directed to a tip electrode for use within an ablation catheter, with improved electrode/tissue interface temperature measurement capability for both end-fire and side-fire ablation modes.
In a first aspect, the invention relates to a tip electrode adapted to be mounted to a catheter for providing electrical energy to biological tissue. The tip electrode includes a distal-end portion, a proximal-end portion contiguous with the distal-end portion, at least one distal-end thermal sensor electrically connected to the distal-end portion, and at least one proximal-end thermal sensor electrically connected to the proximal-end portion.
In detailed aspects, the distal-end portion is substantially dome-shaped and the at least one distal-end thermal sensor is connected near the apex of the dome and the proximal-end portion is substantially cylindrical shaped and the proximal-end thermal sensor is connected near the surface of the proximal-end portion. In a more detailed facet, the tip electrode includes a plurality of proximal-end thermal sensors connected at distinct points around a circumference of the proximal-end portion. In another detailed facet, the distal-end portion and the proximal-end portion are formed of a first metallic material and the at least one distal-end thermal sensor includes a first electrical lead connected to the distal-end portion, the first lead formed of a second metallic material different than the first metallic material and having a Seebeck coefficient relative the first metallic material and a second electrical lead connected to the tip electrode, the second lead formed of a third metallic material and having a Seebeck coefficient relative the first metallic material. The ratio of the magnitude of the Seebeck coefficient of the second metallic material relative to the first metallic material and the magnitude of the Seebeck coefficient of the third metallic material relative to the first metallic material is at least ten to one.
In yet another detailed aspect, the distal-end portion and the proximal-end portion are formed of a first metallic material and the at least one proximal-end thermal sensor includes a first electrical lead connected to the distal-end portion, the first lead formed of a second metallic material different than the first metallic material and having a Seebeck coefficient relative the first metallic material and a second electrical lead connected to the tip electrode, the second lead formed of a third metallic material and having a Seebeck coefficient relative the first metallic material. The ratio of the magnitude of the Seebeck coefficient of the second metallic material relative to the first metallic material and the magnitude of the Seebeck coefficient of the third metallic material relative to the first metallic material is at least ten to one.
In a second facet, the invention is related to a tip electrode adapted to be mounted at the distal-end of an elongated catheter for ablating biological tissue. The biological tissue is located in a biological structure in which fluids flow past the tissue to be ablated. The electrode includes a dome-shaped distal-end portion, a cylindrical shaped proximal-end portion contiguous with the distal-end portion, a tip thermal sensor electrically connected to the distal-end portion and at least one peripheral thermal sensor electrically connected near the surface of the proximal-end portion.
In a detailed aspect, the distal-end portion includes a pocket near the apex of the distal-end portion and the first thermal sensor is positioned in the pocket. In another detailed facet, the distal-end portion is solid and carries a tip-sensor bore terminating in a pocket near the apex of the distal-end portion and the tip thermal sensor is positioned in the pocket. In yet another detailed aspect, the proximal-end portion comprises a hollow tube and the at least one peripheral thermal sensor is positioned at the inside surface of the tube. In still another detailed aspect, the tip electrode further includes a hollow core positioned within the hollow tube for feeding through the tip sensor to the distal-end portion. In yet another detailed facet, the proximal-end portion is solid and carries at least one peripheral-sensor bore and the at least one peripheral thermal sensor is positioned in the bore.
In a third facet, the invention relates to an apparatus for delivering energy to biological tissue. The apparatus includes a catheter having a tip electrode formed of a first metallic material. The tip electrode is disposed at a distal end of the catheter and the distal end is adapted to be positioned so that the tip electrode is located proximal the biological tissue. The apparatus further includes a plurality of electrically conductive sensor leads, each individually electrically connected to the tip electrode. One senor lead is electrically connected near the apex of the tip electrode to form an apex sensor junction while each of the remaining sensor leads are electrically connected proximal the apex to form a peripheral sensor junction. Each sensor junction has a temperature-dependent voltage associated therewith. The apparatus further includes an electrically conductive common lead electrically connected to the tip electrode to form a common junction. The common lead is formed of a second metallic material such that substantially no temperature-dependent voltage is associated with the common junction.
In a detailed facet, each of the sensor leads is formed of a metallic material different than the first metallic material and each metallic material has a known Seebeck coefficient relative to the first metallic material. In another detailed aspect, the ratio of the magnitude of the Seebeck coefficient of the sensor lead metallic material relative to the first metallic material and the magnitude of the Seebeck coefficient of the common lead metallic material relative to the first metallic material is at least ten to one. In another detailed aspect, there are four peripheral sensor junctions and the peripheral sensor junctions are connected to the tip electrode approximately 90xc2x0 apart around a circumference of the tip electrode.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.